The present invention relates to UHF tuners, and more particularly, to UHF tuners of the type which are arranged to provide a separately detented position of the selected shaft for each of the UHF stations in the UHF television band.
Various arrangements have been heretofore proposed in conjunction with a UHF tuner of the continuously variable type to provide a separately detented position for the selector shaft of the UHF tuner for each one of the sevently UHF stations in the UHF television band. One such arrangement is shown, for example, in U.S. Pat. No. 3,774,459 wherein the selector shaft is mounted in the front wall of a bracket, and the fine tuning shaft positioned inside the station selector shaft is mounted on a rear wall of the bracket. The detent positions are established by means of a detent spring which engages detent lobes formed integrally with the selector shaft at a point intermediate the two supporting walls of the bracket. This detent spring exerts a side thrust on the rear end of the selector which is supported indirectly through the central fine tuning shaft.
There is a problem in such arrangement that if a large tolerance is employed between the station selector shaft and the central fine tuning shaft, the side thrust exerted by the detent spring on the rear end of the selector shaft causes the selector shaft to be slightly eccentric with respect to the fine tuning shaft and the clutch plate carried thereby. Under these conditions an action which may be termed "creep" is produced when the selector shaft is rotated through a number of revolutions as it is when traveling from one end to the UHF television band to the other. This "creep" between the selector shaft and the output gear which is carried by and secured to the fine tuning shaft affects the overall alignment accuracy curve of the UHF tuner since the gear connected to the fine tuning shaft drives the rotor of the continuously variable UHF tuner through a spring loaded gear train. For eliminating such creep action, U.S. Pat. No. 3,844,176 discloses the use of the low friction plastic washer being in the severed play or tolerance. The washer needs to have an inner diameter closely fitting with the fine tuning shaft and an outer diameter closely fitting with the selector shaft. However, the above resolution needs a new preparation of washer part for mounting and still requests a small amount of play for the arrangement. Therefore, it is requested to propose a new and improved method for resolution of such problem by reducing the number of parts in the arrangement and achieving easily mounting operation.
Meanwhile, channel selecting operation in such UHF tuner must be achieved easily and stably by the operator who gives necessary torques to the selector shaft or the fine tuning shaft. Magnitudes of these torques which are concerned with the diameter of operating knobs are predetermined so as to make suitable operation, and are generally selected within ranges of 1.5 to 5.0 kilogram-centimeters for the detent selector shaft and 0.5 to 2.5 kilogram-centimeters for the fine tuning shaft. Usually it is preferred that the torque of the selector shaft is in about 2.5 kilogram-centimeters for obtaining the detent effect and the torque of the fine tuning shaft is in about 0.8 kilogram-centimeters for obtaining stable and smooth rotation of the fine tuning shaft. Since clutching means must be effectively actuated by these torques, the conventional clutch employes a clutch pad intermediate the clutching surfaces for establishing suitable friction therebetween as well as the use of lubricant agents, such as of high velocity oil for reducing the differential of dynamic and static torques. The use of the clutch pad causes other problems and one of disadvantages in the above clutch is to vary the coefficient of friction due to temperatures, for instance, by rising temperature the velocity of lubricant oil is reduced so that the necessary torque for fine tuning operation may be unsteady by reducing the coefficient of friction of the clutch. Therefore, it is requested to propose an improved clutch for the channel selecting shaft mounting arrangement, which is relatively inexpensive and performs operation characteristic to be smooth and stable.
Further, there is a manufacturing problem of counter stopping means in the combination disclosed in the above mentioned U.S. Pat. No. 3,774,459 that for determining location of the stopping lug on the selector shaft, the different selector shaft must be prepared for different channel plans in each countries and therefore, it is requested to provide an improved counter stopping means in which the location of the stop lug is changeable due to the channel plan in different countries, such as seventy positions in U.S. and fifty positions in Japan.